The present invention relates to a rotor arrangement of the general type including a rotor-section, for example a section of a turbine shaft, and a circumferential sealing element around the rotor section, the rotor section being provided with a sealing surface for co-operating with the sealing element.
It is necessary in many rotor applications to provide a moveable seal around at least part of the circumference of a rotor-section, for example to isolate relatively high-pressure and low-pressure stages along the rotor-section. Such circumferential seals are conventionally formed between a sealing surface of the rotor-section and an annular sealing element; the sealing-element should optimally be capable of maintaining the seal whilst nevertheless allowing the rotor-section to rotate, relative to the sealing-element, as freely as possible.
A problem associated with circumferential rotor-seals is that, because the sealing-element is necessarily not fixed to the rotor-section, the rotor-section and the sealing-element may be subject to relative movement during operation of the rotor arrangement. Such relative movement may occur due to eccentric movements of the rotor-section, relative to the sealing-element, and/or as a result of concentric radial growth of the rotor-section, such as “centrifugal growth” caused by the rotational speed of the rotor-section and/or thermal growth caused by high operating temperatures.
Relative movement between the rotor-section and sealing-element can result in severe rubbing between the rotor section and sealing element, along the seal, which can in turn have a critical effect on the wear, performance and ultimately the integrity of the seal.
Various types of circumferential seal-assemblies have been developed to try to address the problem of relative operative movement between a rotor-section and the corresponding circumferential sealing-element. One such type of seal-assembly is the brush-seal assembly, disclosed in general terms in U.S. Pat. No. 3,917,150, which incorporates a brush seal element including an annulus of densely packed, flexible, metal or ceramic bristles that form a circumferential seal around the rotor-section; the bristles project inwardly towards the rotor-section from a rigid annular carrier fixed to the rotor casing and seal against the external surface of the rotor-section.
Generally speaking, these brush-seal assemblies can accommodate a relatively small degree of relative movement between the rotor-section and the fixed annular carrier through flexing of the bristles, whilst nevertheless maintaining the overall integrity of the seal. However, any appreciable relative movement has the effect of subjecting the bristles to a high radial load and the bristles consequently wear quickly, critically affecting sealing performance.
In so-called “floating” brush-seal assemblies, such as those disclosed in US 2006/0210392A1, the annular carrier is mounted to slide with the rotor-section in a radial direction so as to maintain the separation between the annular carrier and rotor-section, thereby significantly reducing any eccentric movement of the rotor-section relative to the annular carrier. However, whilst such floating brush-seal assemblies can reduce radial loading of the bristles specifically as a consequence of eccentric movement of the rotor-section, they do nothing to address the distinct problem of radial loading of bristles due to the concentric radial growth of the rotor-section, so that appreciable concentric growth of the rotor-section may nevertheless result in rapid bristle-wear and consequent poor sealing performance.